Mining Aggregate From Landfills

The inventor hopes a process will provide producers a new aggregate source: plastics.

You know it had to happen sooner or later. As concrete increases its footing in green building, someone had to ask the question: Is there a way to recycle the tons of plastic currently filling landfills?

That's probably why the room was filled at the Research in Progress session at the American Concrete Institute meeting last November in Denver. Tom Balkum presented his research findings from his team's development of a lightweight, high-strength concrete aggregate created from recycled plastics. He has named the product Balkrete Custom Aggregates. From the interest in the room, the timing seems right.

Building green, Leadership in Energy and Environmental Design (LEED) certification, and sustainable design are all hot topics in the construction industry. But one question that always gets asked is, how can concrete be designed to be more responsive to these demands? Recognizing this opportunity, Balkrete searched for a way to create a custom aggregate for use in concrete that could be produced with plastic recovered from the waste stream.

There seems to be an unending source. “The volume of plastics is increasing each year, while recycling rates are steadily declining, as traditional uses of recycled plastics require near 100% purity,” said Balkum. In 2006, almost 50 million tons, or 150 million cubic yards, of plastics will be disposed of in the United States, of which less than 3% will be recycled.

The development team has developed a group of proprietary technologies to process landfill-diverted mixed plastic waste into high-strength, lightweight aggregate. They have developed and tested hundreds of plastic aggregate formulas, focusing on the attributes best for concrete applications.

Bonding with cement

Their goal was to modify the recycled plastic to ensure a solid bond with the cement. They began testing mixes by substituting raw untreated plastic for traditional aggregates and establishing a baseline 28-day compressive strength. They quickly discovered that the slick surface of the untreated plastic allowed for an insufficient bond to the cement paste. Low compression tests resulted.

Their next step was to thermally embed fine sand into the surface of the plastic aggregate. The irregular surface created a natural physical bond to the cement paste and doubled the compressive strength of the finished concrete. Thermoforming the plastic while modifying the surface also creates the opportunity for custom shapes.

To accomplish both design goals, they focused on developing a formulation mix of the recycled plastics. This also enabled the development of the custom performance aggregate.

For example, aggregate made from raw chipped mixed waste plastics was suitable to batch a 700 psi, 50 pcf, low-strength concrete for grouting. By creating aggregate by using mixed high-density waste stream plastics to form high-strength aggregates treated with a proprietary bonding process, they developed a mix design that had a 3500+ psi, 28-day strength at 100 pcf, high-strength structural grade of lightweight concrete.

According to Balkum, the company has recently patented the processes needed to form and modify the plastics into aggregates that ensure an adequate bond to hydraulic cement. He hopes that engineers interested in ecologically friendly concrete will look at his special aggregate.

Not only has Balkum developed a plastic aggregate that expands the performance capabilities of concrete technology, he's about to create a new aggregate business opportunity and a potentially profitable market that diverts millions of tons of non-biodegradable waste annually. “Concrete mixes produced with our custom aggregates achieve 60%-80% post-consumer recycled content,” he says.

Balkum is hoping others share his goal. The startup company is seeking partners in sales, manufacturing, distribution, and finance to take the technology to market.